Successful Fireplaces in Tight Houses

A central location, a tall chimney, and controlled combustion are the keys to a good burn

Cold Hearth Syndrome
But the biggest source of trouble is the location of the
fireplace. Over the past 50 years of residential design,
fireplaces have migrated from the center of the house to a
position against the exterior walls, or even into chases that
are completely outside the house. This causes cold hearth
syndrome, which is the source of most fireplace failures.
The most dramatic effect of a cold hearth is a predictable
blast of cold air when the fireplace doors are opened to build
a fire. Smoke tends to fill the room when someone tries to
light a kindling fire. This is a common, even chronic,
characteristic of North American fireplaces.
The syndrome usually has its origin in the decision to place
a fireplace outside an exterior wall in a frame or brick chase
(Figure 2).
Figure
2. Chimneys built on an outside wall, whether exposed or
boxed with an uninsulated chase, are prone to downdrafting
(top). One solution is to insulate the walls of the chase and
to vent the chase to the interior so warm air can circulate
(middle). The best solution is to locate the chimney properly
in the first place. The ideal location is in the center of the
house (bottom), because the surrounding air will keep the
chimney warm and the chimney will penetrate the roof at its
highest point.
The cold outside air sucks warmth from the fireplace and
chimney structure, causing the temperature of the air in the
flue to drop. When the flue temperature is lower than the house
temperature, air begins to flow down the chimney and onto the
hearth. This is called a "cold backdraft" and contrary to
common belief, it does not happen because cold air is heavy and
falls down the chimney. The air is not falling — it is
being sucked down by the house.
Just as hot exhaust in a chimney produces a pressure
difference called a "draft," so the relatively warm air in a
house produces a pressure difference called "stack effect." The
buoyant warm air rises, producing a slight low pressure zone
downstairs and higher pressure upstairs. Since most fireplaces
are installed on lower floors, they experience negative
pressure due to stack effect when it is cold outside. As soon
as the air in the chimney falls below room temperature, the
house becomes a better chimney than the chimney itself, and a
cold backdraft gets started. The backdraft tends to stabilize
because as the chimney becomes full of cold air, it cannot
produce any draft to resist the suction of the house.

Tall stack effect.

A
similar problem is caused by chimneys that fail to extend
higher than all of the living space in a house. A chimney that
is not tall enough competes with the living space above it to
establish the dominant draft (Figure 3).
Figure
3. Chimneys built on outside walls are often too short
to counter the house’s stack effect and are prone to
downdrafts. Again, moving the chimney closer to the center of
the house ensures a tall chimney with a strong
draft.
If the upper part of a house envelope leaks enough air
through windows, attic access hatches, and wall penetrations,
the house will again act as a better chimney than the chimney
itself. In these cases, the house is said to have a "taller
effective stack" than the chimney. Air will tend to flow down
through the chimney, then loop through the house to exit
through the attic or upper-story wall leaks. To avoid these
problems, chimney tops should always penetrate the highest
section of the conditioned living space.
In all cases, the cold hearth syndrome has two necessary
ingredients without which it will not occur: a misplaced
chimney and a fireplace located low in the house. If we could
move the fireplace and its chimney towards the center of a
house, the syndrome would vanish. Or we could move the
fireplace to the highest floor of the house where the higher
pressure caused by rising warmer air would ensure a good
draft.
Unfortunately, moving a problem fireplace is not practical
after it’s been built, but you can still prevent cold
hearth syndrome by keeping the chimney from falling below room
temperature. One way to do this is to trick the fireplace into
thinking it is inside. This requires building a sealed,
insulated chase that thermally matches the house wall
construction. The chase should be vented to the inside so that
warm house air circulating in it will keep it at about house
temperature.
A better solution is to design out cold hearth syndrome at
the planning stage by bringing the fireplace and chimney in
from the cold. Ideally the fireplace should be located
centrally, in the heart of the home, so that the chimney will
penetrate the roof closer to its highest point. This makes for
a tall chimney with a large temperature differential between
combustion exhaust and outside air — the two ingredients
that make for the most reliable and stable draft. Straight
venting systems also work better, so elbows and offsets in the
chimney should be avoided.